基金Acknowledgements. This paper was written following an invitation from Riyu LU （Editor, AAS）. This study was supported by the National Key Basic Research Program of China （Grant No. 2014CB953902）, the National Key Research and Development Program of China （Grant No. 2016YFA0600603）, and the National Natural Science Foundation of China （Grant Nos. 41475081, 41275081 and 41530425）.